A complementary thin film transistor (i.e., comprising both an N-type thin film transistor and a P-type thin film transistor) typically adopts low-temperature polysilicon (LTPS) as semiconductor layer material. But, since the process for the low-temperature polysilicon is complicated and has poor controllability and poor uniformity, the low-temperature polysilicon has not been used in a display device having a large area of over 6-generation line.